Otherwise has been in English use for a considerable length of time, as an adverb (from the 13th century), an adjective (from the 14th century), and as a pronoun (since the 15th century). It may be traced back to the Old English (on) ōthre wīsan ("in another manner). The adverb sense that Trump was employing is most often used to indicate "in other respects."

In "Margery," a girl in white on a sofa, the face is not the best part of an other wise well-painted portrait.— The Scotsman (Edinburgh, Scotland), 28 Feb. 1913

I caught bass continuously but their individual capture is not worth recording except to say that they absorbed the best part of an otherwise endless night.— Anthony Pearson, The Guardian (London, UK), 18 Dec. 1971

This hit parade from the past is the best part of an otherwise tedious tale of a Liverpool docker's midlife crisis, which he is handling rather badly.— The New York Times, 19 Aug. 1990